This invention relates generally to phased array antennas and more particularly to a method and apparatus for enhancing the instantaneous bandwidth of a phased array antenna system.
A phased array antenna is comprised of a plurality of fixed elements which can be electronically controlled to steer a radiated beam, or receive an incident beam, at a desired angle xcex8 relative to the antenna boresight. The steering angle xcex8 can be controlled by adjusting the relative phase shift of the excitation signal at each of the elements defining the antenna aperture. The set of phase shift coefficients required by the plurality of antenna elements to achieve a certain steering angle xcex8 is frequently referred to as the xe2x80x9cphase taperxe2x80x9d for that angle (e.g., See Antenna Engineering Handbook by R. C. Johnson, Chapter 20 xe2x80x9cPhased Arraysxe2x80x9d).
A typical phased array antenna can transmit and/or receive. The transmit and receive operations are generally reciprocal, i.e., identical except for opposite directions of radiation. For clarity of explanation, most of the discussion hereinafter will focus on the receive mode of operation, but it should be understood that the discussion is generally equally relevant to the transmit mode.
Beam steering is typically accomplished by appropriately phase shifting respective excitation signals at the plurality of antenna elements. More particularly, a received beam incident on the antenna at an angle xcex8 produces excitation signals at the plurality of elements, which, when properly phase shifted in accordance with a phase taper appropriate to the angle xcex8, can be added coherently to produce an antenna input/output signal. Unfortunately, the phase taper required to steer to a specific angle is dependent on the frequency of the beam signal. As a consequence, the signal bandwidth for any fixed taper, i.e., xe2x80x9cinstantaneous bandwidthxe2x80x9d, is limited (e.g., See Radar Handbook by M. I. Skolnick, Section 7.7 xe2x80x9cBandwidth of Phased Arraysxe2x80x9d).
It is well recognized that xe2x80x9cinstantaneous bandwidthxe2x80x9d varies inversely to the size of the array and the magnitude of the steering angle from boresight. That is, the larger the array and/or the larger the steering angle, the lower the instantaneous bandwidth. Therefore, large phased arrays are generally considered unsuitable for very high bandwidth applications; e.g., extremely high data rate communications and extremely fine range resolution radar.
Recent research has examined the use of true time delay, rather than phase shifting, to steer an antenna beam. The time delays required to steer a beam to a specified angle do not vary as a function of signal frequency and therefore a true time delay steered array, in theory, has infinite bandwidth. Unfortunately, true time delay is very difficult and costly to implement. A typical true time delay embodiment would require switched lines behind each array element which cannot, in some applications, be readily accommodated. Moreover, differences in line length across the antenna aperture are likely to produce differential attenuation which can significantly distort radiation pattern side lobes. Although such distortion can be minimized by incorporating variable gain amplifiers in each switched line, such a solution further increases cost and complexity.
The present invention is directed to a phased array antenna system, and method of operation, designed to exhibit a wider instantaneous bandwidth than known phased array systems.
A phased array antenna system in accordance with the present invention is configured to transfer signal energy between an antenna array and a source/target via multiple concurrent beams respectively centered in different frequency channels, i.e., different slices of the frequency spectrum. Each of the plurality of antenna elements will thus receive a composite signal which can then be band pass filtered to separate the different frequency signal components. By separating the signal components, a different phase taper can be applied to each signal component thereby enabling coherent signal energy to be derived from the plurality of elements. The derived signal energy can then be combined for the multiple beams to produce the antenna input/output signal.
The use of multiple beams to concurrently carry a common information signal provides a low cost, high performance technique of achieving a wider bandwidth phased array antenna system. The wider bandwidth enables such a system to transmit and receive high frequency components of pulsed signals which heretofore could not be comparably handled. Thus, embodiments of the invention are suitable for use in very high band width applications.